1. Field of the Invention
The present invention relates to microwave tubes and, more particularly, to longitudinal interaction tubes such as travelling-wave tubes or klystrons. It can even be applied to gyrotrons. The operation of these tubes is based on an exchange of energy between an electron beam and microwave energy. The electron beam is emitted in a gun, by a cathode. The gun is placed at the input of a tubular interaction space. The electron beam is long and thin. It crosses the interaction space. A focusing device surrounds the interaction space and confines the electrons of the beam to desired paths.
Microwave energy is injected into the interaction space. It interacts with the electron beam and is amplified. The microwave energy is extracted by an appropriate device at an output of the interaction space. The electron beam ends its travel in a collector located at an output of the interaction space.
The interaction space comprises a microwave circuit which is generally either a helix delay line in the case of a travelling-wave tube or a sequence of resonant cavities in the case of a klystron. The interaction space is taken to a potential that is generally a ground.
After having yielded a part of its energy to the microwave energy, the electron beam still has substantial kinetic energy when it penetrates the collector. The collector dissipates this energy in the form of heat. The collector, gun and interaction space and brought together in a vacuum envelope that is positioned on a flange mounted on a cold part. The flange contacts a heat-conducting member located between the envelope and the cold part which is cooled by natural or forced convection, by the circulation of a fluid or by radiation. The cooling is done by thermal conduction through the flange. Since the collector is the part that dissipates the greatest amount of heat, it should be especially well cooled. The envelope, at the level of the collector, is held in a clamp that generally forms one piece with the flange.
2. Description of the Prior Art
The efficiency of prior art tubes is generally low, and the energy of the electrons penetrating the collector is greater than that of the microwave collected at output of the tube. To increase this efficiency, it has been necessary to use a so-called "depressed" collector. It is sought to recover a part of the energy of the electrons of the beam by decelerating them. A depressed collector is carried to an intermediate potential between the potential of the cathode and that of the interaction space. The collector may comprise an electrode or several successive electrodes. When there are several electrodes, they are carried to potentials that decrease with distance from the interaction space. The collector then has several stages. The use of a depressed collector contributes to increasing the efficiency of the microwave tube and to reducing the difficulties encountered in removing the heat. The power dissipated as heat on the collector can be reduced to a value close to that of the microwave power at the output of the tube.
This power very often continues to remain high and its discharge calls for a substantial exchange surface area between the flange and the cold part. This may raise problems if it is desired to restrict the space occupied by the tube.
The collector is formed by one or more electrodes in the form of cups, drilled in their central part, insulated from the conductive envelope by non-conductive rods. There may be a difference in temperature of some degrees centigrade between the internal and the external walls of the electrodes. There may be a difference of some tens of degrees between the face of the rods in contact with the electrodes and the face in contact with the envelope. There may be a difference of some hundreds of degrees between the face of the flange in contact with the envelope and the face in contact with the cold part.
Another known approach used to improve the cooling of the tube and notably that of the collector is to integrate heat channels between the electrodes and the envelope of the collector. However, this approach leads to a heavy and bulky tube, the operation of which may border on inefficiency.